This invention relates generally to a method of manufacturing thermoplastic sheet and more particularly, but not by way of limitation, to a thermoplastic sheet with a foamed core and two or more solid outer thermoplastic skins.
Heretofore, solid thermoplastic sheet products made out of ABS polystyrene, polyethylene, poly vinyl chloride sheet products, or the like, have been standard in the industry. The incorporation of blowing agents into the thermoplastic materials for gravity reduction is old in the art. In injection molding and other converting processes, it has been a common practice to use blowing agents in the thermoplastic material for achieving either cost reduction, weight reduction, or other special functions like the elimination of molding sinks, etc.
In extrusion processes, various methods have used blowing agents for producing special foamed products such as foamed styrene for egg carton sheets, wood simulation, etc.
The problems with using blowing agents in thermoplastic materials for producing extruded sheet products have been providing adequate surface appearance and the physical properties of the foam plastic sheet has not met the necessary requirements when compared to solid thermoplastic sheet for engineering applications.
The co-extrusion of thermoplastic sheets having solid outer skins with a foamed core has offered a real potential for overcoming the problem of surface appearance of the sheet and providing the necessary impact properties required of the end product.
In U.S. Pat. No. 3,855,376 to Ono et al, a process for making structural material having a foamed inner core is disclosed. In this process, flat surface layers consisting of molten resins and an inner layer consisting of a molten resin having a blowing agent therein are introduced through separate entrances in a sheet-forming die. The surface layers and the inner layer are joined in the die without mixing the layers together. Pressure is applied to the layers to shape the surfaces smooth prior to discharging the formed sheet from the die wherein the pressure is removed and the inner layer resin is allowed to foam. This invention requires the use of an expensive die for receiving separate layers of the molten resin along with the maintenance problem of having to continuously and for each layer adjust the overall width of the die which in practice varies typically from 50 inches to 120 inches so that the required thickness of the sheet is achieved.
There are additional methods of forming multiple layer plastic sheets, tubing, conduits, or the like and in particular U.S. Pat. No. 3,773,877 to Baker et al, U.S. Pat. No. 3,857,914 to Aishima et al, and U.S. Pat. No. 3,476,627 to Squires. The subject invention is distinguishable over these patents, and the patent to Ono et al and provides an economical method of forming a thermoplastic sheet with solid skins and a foamed core.